1. Field of the Invention
This invention relates to filling compounds or sealing compounds for electrical components, such as cables, light-wave conductors, terminal strips or plugs. The filling compounds consist of a polymer solution in which pyrogenic silica is dispersed and which may also contain hollow bodies (e.g., hollow beads). The hollow bodies are optional when the filling compounds are used for light-wave conductors. The filling compounds protect cables and light-wave conductors against the penetration of contaminants, particularly water, and also against mechanical damage during laying or through the effects of temperature.
2. Description of Related Art
It is known that communications cables and also light-wave conductors encounter the danger that any moisture penetrating at a given point spreads out axially and not only reduces the serviceability of the line, but also has a corrosive effect thereon. There has never been any shortage of attempts to overcome this problem by the provision of suitable sealing compounds. For example, German Pat. No. 3150909 describes a filling compound consisting of a viscous water-immiscible substance and elastic hollow bodies finely dispersed therein, the hollow bodies being of such a nature that they can be compressed to a smaller volume by relatively strong forces and thereafter reassume their original volume as the forces diminish.
Although such filling compounds afford a number of advantages by virtue of their relatively low dielectric constant, there is nevertheless a danger at relatively high operating temperatures that the filling compound may escape from the cable through sheath damage or that at least oil leakages may occur. These effects may be counteracted to a certain extent by increasing the proportion of elastic hollow bodies to such an extent that the compounds are only able to flow under pressure, i.e., when the elastic hollow bodies are compressed to a smaller volume. Although it is possible in this way to make cables having slightly better properties, the procedure involved is complicated on account of the high pressures under which the filling conmpound has to be introduced.
To improve the thermal stability under load of the filling compounds according to German Pat. No. 3150909, attempts have been made to stabilize the hollow beads be cross-linking. Thus, it is proposed in German Pat. No. 3213783 to cross-link the beads by electron beam treatment. Although the thermal stability limit of the hollow bodies is thus increased by about 10.degree. C., the filling materials thus treated still show very poor thermal stability under load.
Accordingly, there is a need for filling materials for cables or light-wave conductors which do not escape from damaged parts of a cable even at operating temperatures of about 80.degree. C. or slightly above and which show plastic flow over the entire temperature range of importance in practice, i.e., the filling materials are unable to flow up to a certain shear strain. Nevertheless, the filling materials are intended to show the favorable properties of the known materials containing hollow bodies, particularly in that their dielectric constant is &lt;2.0. Finally, the filling compounds are intended to be processible under the conditions normally applied in cable manufacture.